Wafer bumping is a method of packaging silicon for high density and high frequency applications. With the bumping process being constantly improved and cost decreasing, this interconnect method is becoming more popular. As a result, it is increasingly important to identify defective wafer bumps (e.g. too large, small or missing bumps) in order to prevent device failures, which may lead to wasted time testing the defective die and packaging expense.
As known in the art, most optical systems, such as confocal, interferometry, or moiré, which perform 3D criteria inspection of wafer bumps (e.g. extract information related to bump diameter, height, intensity, area, position, as well as defect detection) are typically required to remain completely still in order to acquire a plurality of images of a relatively small wafer area. This results in significant overhead in accelerating, traveling and decelerating times and waiting for the motion to settle down after the system has moved to a new position. Another drawback in this case is that the motion system is heavy and expensive as it is highly susceptible to excessively high acceleration and deceleration, self induced vibrations, as well as wear and tear.
In addition to the above drawbacks, another disadvantage of such prior art scanning systems is that the length of exposure is the same regardless of the portion of the object being scanned. As a result, if the length of exposure is optimised for one part of the object, other portions of the object may end up being scanned in a suboptimal manner. For example, in the case of a wafer bump type substrate the wafer surface is substantially flat and highly reflective. On the other hand the bumps, having generally curved upper surfaces, disperse light. The resulting contrast between the upper surfaces of the bumps and the wafer surface is very high and as a result use of the same length of exposure for the top of the wafer bumps and the wafer surface leads to suboptimal results. As a result, the 3D data (the height and intensity measurements in particular) collected and represented on the object image is generally of poor quality and accuracy.